1. Field of the Invention
This invention relates to the design and fabrication of airfoil shapes, and particularly to composite fiber wound large scale wind turbine rotor blades. More particularly, the invention provides a method for avoiding the problem of bridging which occurs when the composite fiber is wound over a concave mandrel surface to form the airfoil shape. The fibers, being under tension during the winding process, will not follow a concave contour or valley of the surface, but will form a bridge, resulting in the occurrence of voids in the surface which weaken the blade structure.
2. Description of the Prior Art
Techniques for fabrication of airfoils such as propeller and rotor blades are well known in the art, and include the use of wood, wood laminates, various metals, and more recently composite materials such as fiberglass. Very large rotor blades, such as those used in wind driven turbine generators, present unique problems due to their very large size, up to 300 feet in combined length. A preferred cost and weight saving technique for fabricating these blades is by a process that involves winding fibers onto a mandrel. A band or group of parallel resin-impregnated filaments is wound onto a slowly rotating mandrel. The band typically is about 2 inches wide, and composed of a plurality of rovings, each from a separate spool. Each roving consists of a large number of filaments, so that the band contains many thousands of separate glass filaments. The payout guide is positioned during mandrel rotation to produce the desired band path on the mandrel. Bridging, or winding over a concave area of the mandrel, does not occur on cylindrical shapes, but can be expected on a wind turbine blade because of blade twist and its root-to-tip thickness characteristic. With a filament winding angle of 30 to 40 degrees, the concave shape also appears along the desired band path. If a section is cut along the band path, the section is bridged if there is a void between the mandrel and the fiber or filament pulled tightly across it.
The most visible problem caused by bridging is voids, which weaken the structure. The voids may be filled with glass and resin to make a solid structure, but this adds substantial weight at considerable extra cost. Bridging can produce poor fiber compaction, thus increasing the resin-to-glass ratio and lowering its strength. Loss of fiber control means that an unsupported band will tend to form a rope, or to separate.
The angle of winding of the fibers is determined as required by the specific shape and loads on the blade, and the angle may be varied along the longitudinal axis of the blade. Further, conventional winding techniques normally involve multiple winding passes whereby layers of fibers are built up to form the airfoil. In some applications specific portions of the airfoil or blade may contain more layers of fibers than others, e.g., in rotor blades it is common to apply many more layers of fiber to the inboard or hub end than to the outboard end to enhance structural rigidity and to absorb loads.
In many applications a so-called winding or adapter ring is used at the end of the blades, the fibers being wrapped about the ring during fabrication and the fibers being cut off at the end of the blade after fabrication. Again this technique is well known.
In some applications the fibers in different passes may be of different compositions, and different passes may use fibers of varying thicknesses, or different spacings, or different angles. A common technique is to perform one winding pass on a right-hand helical path, with the next pass being on a left-hand helical path.
For large blades a solid surface is generally used as the mandrel over which the fibers are wound. The mandrel may be, for example, a plywood frame covered with wire cloth and a plaster filler, or it may be aluminum or plastic. In some applications a spar section is located inside the rotor or airfoil for added strength, with mandrel sections located adjacent the spar. Upon fabrication, the mandrel may be removed from the inside of the airfoil, or it may be left in place to act as a structural reinforcement.
Although the invention will be described with respect to glass fibers coated with resin or other epoxy matrix, it is apparent that other types of fibers and/or matrices are equally applicable, and that single or multiple fibers may be used in practicing the invention.
Bridging may be prevented in some cases by varying the winding angle, but this is not always practical since changing the winding angle changes the strength and load absorbing characteristics of the rotor. Another solution is to modify the mandrel design, and yet another solution is to determine in advance from the design geometry the localized areas of the mandrel where bridging will occur, and adjust the shape of the design geometry and the mandrel to avoid bridging. In other words, fixing an airfoil to avoid bridging means slightly changing the mandrel shape so it is not concave along any band path. Airfoil changes resulting from bridge-fixing are primarily near the trailing edge of root stations, resulting in a negligible impact on aerodynamic performance.
It is therefore an object of this invention to provide a method which avoids or reduces bridging in the fabrication of large scale fiber wound rotor blades.
Another object of this invention is a method for determining where bridging will occur when a composite fiber is wound over a mandrel or other contoured structure.
A further object of this invention is a method for making minor changes in the shape of the mandrel or structure upon which a fiber composite is wound to avoid bridging.